Proposed herein is a population-based case-control study of breast cancer in women in relation to exposure to Extremely Low Frequency (ELF) magnetic fields and light-at-night (LAN). Eligible as cases will be women diagnosed with breast cancer under the age of 75 in a two-county area of northwestern Washington State from July 1992 - December 1994. A random sample of 800 will be enrolled in the study. Control women, without breast cancer, equal in number and comparable in age and race, will be randomly chosen from the same two-county area by random digit dialing. A personal interview will be conducted will all subjects. ELF magnetic fields will be measured directly in each subject's current residence for two days on a 24-hour basis. These measurements will be repeated during one weekend day and during each of the four seasons for a 10% sample of subjects. The wiring configuration around each residence within a larger three-county area during the past 10 years will be mapped, as will the wiring configuration around the current residence locations of non-respondents. Measurements and wiring configuration data will be used to derive cumulative exposure classifications. Ambient light intensity will be measured in each subject's bedroom as part of the 24-hour ELF magnetic field measurement process. In addition to these two major exposures of interest, the independent effects on the risk of breast cancer of previously established and recently postulated factors will be investigated, particularly in regard to potential interactions with ELF magnetic field and LAN exposures.